Frequency synthesis refers to the generation of a signal of precise frequency by the use of one or more reference frequencies. A commercially successful method employs a control system known as a phase locked loop (PLL). This method utilizes negative feedback to match the phase of the frequency divided output of a controlled oscillator to that of a reference frequency. The output is an adjustable multiple of the reference frequency. This multible is usually an integer or ratio of simple integers, and only a finite number of output frequencies normally are available.
The method and apparatus provides frequency accuracy proportional to the accuracy of the reference sources, usually crystal oscillators. While the output frequencies are changeable only in discrete steps, the modern phase locked synthesizers are capable of so many different frequencies that they can simulate, but not attain, the function of continuous tunable oscillators. In many applications this stepwise tuning is acceptable, but in others the ability to generate an arbitrary frequency is desirable. One of those applications is that of a local oscillator for radio receivers and transmitters, where phase locked synthesis has recently supplanted free running tunable oscillators as the primary tuning technique. Phase lock has the advantages of precise control of frequency, extremely low drift and compatibility with computer control.
Despite these advantages, phase lock does have the disadvantages of discrete frequency stepping and also considerable difficulty in attaining accurate frequency modulation. These weaknesses motivated the development of the precision Frequency Locked Loop of the present invention.
The frequency locked loop (FLL) is similar, in concept, to the PLL. Like the PLL, the system is a negative feedback control system that drives a voltage controlled oscillator (VCO) frequency to be N times a reference frequency. For this to hold exactly, the loop filter must have infinite DC gain. The PLL did not require this condition because of the integrating action of the VCO with respect to phase gives infinite DC gain. So long as the PLL has nonzero DC gain, the loop has infinite DC gain. With frequency as the control variable, infinite gain must be supplied by the filter to drive the error to zero.
An advantage of the FLL is that it inherently has 90 degrees less loop phase shift than the PLL. This is due to the VCO being merely a gain block, rather than an integrator. Consequently 90 degrees more phase shift is available for filtering. This allows better suppression of noise in the detector output than the PLL.
A FLL frequency syntheziser, despite its advantages, has not found widespread use because of the requirement of a linear frequecy detector. This requirement has been provided by the delta sigma frequency detector disclosed in U.S. Pat. No. 4,758,821. Accordingly, it is an object of the present invention to provide a highly accurate, continuously tuneable frequency synthesizer.
It is another object of the present invention to provide a linear frequency detector in a FLL thus making available in industry a frequency synthesizer of widespread application.